Agricultural skin grafting

ABSTRACT

A method of forming a material structure from structural units contained within a liquid solution in a spray head is described. The liquid solution includes a solvent and a solute, the solute comprising a plurality of the structural units, the structural units including monomer units, oligomer units, or combinations thereof. The method comprises forming droplets of the liquid solution including the structural units, and spraying the droplets on a substrate, thereby substantially increasing the reactivity of the structural units within the droplets relative to the structural units within the liquid solution in the spray head. The increase in reactivity can result from the droplets containing an excess of a particular ion, the ion excess resulting from a voltage applied to conductive walls of the device which dispenses the droplets. The material structure is then formed on the substrate from the more highly reactive structural units within the droplets.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/953,504, filed Jul. 29, 2013, now allowed, the entire contents ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

Described herein are methods and an apparatus for depositing material ona substrate, in particular for use in post harvest preservation ofproduce.

BACKGROUND

The two leading causes of produce spoilage are water loss viaevaporation through the produce surface, and oxidation via reaction withoxygen gas that has diffused into the produce. Conventional approachesto preventing spoilage such as refrigeration or special packaging arecostly to implement. Refrigeration requires expensive capital equipment,must be actively managed, has constant energy demands while operating,and can cause damage or quality loss to the produce. Special packagingrequires expensive equipment, must be actively managed, and consumespackaging materials. Even with refrigeration and special packaging, thehandling and transportation of the produce causes surface abrasion orbruising that is aesthetically displeasing to the consumer and serves aspoints of ingress for bacteria and fungi.

As a natural defense against spoilage, the aerial surfaces of all landplants are covered by a thin, highly cross-linked polyester known ascutin. Depositing an edible coating atop this cutin layer has been shownto reduce post harvest water loss and oxidation while helping resistsurface abrasion. However, it is difficult to generate these coatingsfrom non-toxic solvents with optimal diffusion barrier properties, asthe films need to be processable (i.e. they must be highly soluble).Thus, conventional application of most edible coatings requires twosteps; the deposition of the film, followed by a separate crosslinkingstep. These multiple processing steps make the use of edible coatingsdifficult to commercialize and increase the handling requirements of theproduce.

SUMMARY

This specification describes methods and apparatus for depositingmaterial on or applying a coating to a substrate. The coating can, forexample, be an edible coating for produce that is grown at the site ofdeposition using only a single processing step, resulting in reducedwater loss and oxidation of the produce while adding mechanicalintegrity to the surface. While coatings deposited by other methods aretypically unbound to the coated surface, the methods and apparatusdescribed herein can result in a coating which is covalently bonded toat least a portion of the surface. This may result in improved coatingproperties, such as higher durability and tighter control of coatingpermeability and thickness. Furthermore, layer by layer deposition ofsuch coatings which bind to previously deposited material may be used tobuild more sophisticated three-dimensional structures (i.e. used as inkfor three dimensional printing). In addition, the methods describedherein enable the increase or decrease of ion concentrations within asolution immediately prior to contact of the solution with thesubstrate, which is useful in applications such as antimicrobial washes.

In a first aspect, a method of forming a material structure fromstructural units contained within a liquid solution in a spray head isdescribed. The liquid solution includes a solvent and a solute, thesolute comprising a plurality of the structural units, the structuralunits including monomer units, oligomer units, or combinations thereof.The method comprises forming droplets of the liquid solution includingthe structural units, and spraying the droplets on a substrate, therebysubstantially increasing the reactivity of the structural units withinthe droplets relative to the structural units within the liquid solutionin the spray head. The material structure is then formed on thesubstrate from the structural units within the droplets.

In a second aspect, a method of forming a coating over a surface of asubstrate from structural units contained within a liquid solution in aspray head is described. The spray head comprises a dispensing devicehaving an electrically conductive wall, and the liquid solution includesa solvent and a solute, the solute comprising a plurality of thestructural units. The structural units in the solute include monomerunits, oligomer units, or combinations thereof. The method comprisesapplying a voltage between the wall of the dispensing device and eitheran annular electrode of the spray head or ground, thereby causing asurface of the liquid solution adjacent to the dispensing device tobecome electrically charged. The method also includes forming dropletsfrom liquid solution at the electrically charged surface, causing thedroplets to have an electrical charge, and directing the droplets to thesurface of the substrate, wherein the droplets include a plurality ofthe structural units of the solute. The method further includes formingthe coating over the surface of the substrate from the plurality of thestructural units in the droplets.

In a third aspect, a method of catalyzing covalent bonding of structuralunits on a surface of a substrate is described. The structural unitscomprise monomer units, oligomer units, or combinations thereof. Themethod comprises providing a spray head which includes a dispensingdevice having an electrically conductive wall, the spray head containinga liquid solution which includes a solvent. The method further includesapplying a voltage between the wall of the dispensing device and eithera ground or an annular electrode of the spray head, wherein the appliedvoltage causes a surface of the liquid solution adjacent to thedispensing device to have a net electrical charge. The method alsoincludes forming droplets from liquid solution at the electricallycharged surface adjacent to the dispensing device, causing the dropletsto have a net electrical charge, and spraying the droplets on thesurface of the substrate. The method further includes allowing solventin the droplets to at least partially evaporate, wherein the netelectrical charge of the droplets causes an ion concentration of theliquid solution contained within the droplets to change as the solventevaporates.

In a fourth aspect, a method of modifying an ion concentration of aliquid solution is described. The method includes providing an assemblycomprising a reservoir coupled to a dispensing device, the dispensingdevice comprising an electrically conductive wall, the reservoircontaining the liquid solution. The method also includes applying avoltage between the wall of the dispensing device and either a ground oran annular electrode of the assembly, thereby causing a surface of theliquid solution adjacent to the dispensing device to have a netelectrical charge. The method further includes forming droplets fromliquid solution at the electrically charged surface adjacent to thedispensing device, causing the droplets to have a net electrical charge,and allowing the droplets to partially evaporate. The partialevaporation of the droplets causes the ion concentration of the solutionin the droplets to differ from the ion concentration of the solution inthe reservoir.

In a fifth aspect, an assembly adapted for forming a material structurefrom a plurality of structural units is described, where the structuralunits comprise monomer units, oligomer units, or combinations thereof.The assembly includes a spray head adapted to hold a liquid solutionincluding a solvent and a solute, the solute comprising the structuralunits in a non-reactive state or in a state of low reactivity, such thatthe structural units do not substantially bond with one another, thespray head having an attached dispensing device whose walls areelectrically conductive. The assembly also includes a voltage supplyhaving a first terminal and a second terminal, with the first terminalelectrically connected to the walls of the dispensing device and thesecond terminal electrically connected either to ground or to an annularelectrode of the spray head. The application of a voltage from thevoltage supply causes electrically charged droplets of the liquidsolution to be emitted from the dispensing device, thereby changing thepH of the droplets as solvent in the droplets evaporates.

The methods, assemblies, and apparatuses described herein can eachinclude one or more of the following features. Substantially increasingthe reactivity of the structural units can enable the structural unitsto covalently bond with one another or with the substrate at a higherrate than before the increase in the reactivity. The material structurecan be a coating, and the coating can be formed over an entirety of andcompletely surround the substrate. The substrate can be edible tohumans, and the coating can be an edible coating. The coating can beconfigured to prevent or suppress water loss or uptake by the substrate,volatile loss or uptake by the substrate, oxidation via reaction withoxygen gas that can diffuse into the substrate, or surface abrasion. Thematerial structure can be a coating, and the coating can be formed overa portion of the substrate. The spray head can include a capillaryelectrically connected to a first terminal of a voltage supply, andvoltage provided by the voltage supply can cause a surface of the liquidsolution adjacent to the capillary to have a net electrical charge. Thespray head can further include an annular electrode, and a secondterminal of the voltage supply can be electrically connected to theannular electrode.

A second terminal of the voltage supply can be electrically connected toan electrical ground. The droplets can be formed from liquid solution atthe surface adjacent to the capillary, causing the droplets to have anet electrical charge. The method can further comprise allowing solventfrom the droplets to at least partially evaporate, wherein the netelectrical charge of the droplets causes the liquid solution containedwithin the droplets to have a different ion concentration than theliquid solution contained within the spray head after the at leastpartial evaporation of the solvent. The difference in ion concentrationbetween the liquid solution contained within the droplets and the liquidsolution contained within the spray head can cause the structural unitsin the droplets to be substantially more reactive than the structuralunits contained within the spray head. Forming the material structurecan comprise polymerization of the structural units, wherein thepolymerization occurs directly on or adjacent to the surface of thesubstrate.

The forming of the coating can comprise polymerization of the structuralunits, wherein the polymerization occurs directly on or adjacent to thesurface of the substrate. The structural units can be monofunctionalmolecules, and forming the coating can comprise forming a substantiallymonolayer coating of the monofunctional molecules. The forming of thecoating can further comprise causing a plurality of the monofunctionalmolecules to each form a covalent bond to a bonding site on the surfaceof the substrate. The method can further include allowing solvent fromthe droplets to at least partially evaporate, wherein the net electricalcharge of the droplets causes the liquid solution contained within thedroplets to have a different ion concentration than the liquid solutioncontained within the spray head after the at least partial evaporationof the solvent. The difference in ion concentration between the liquidsolution contained within the droplets and the liquid solution containedwithin the spray head can cause the structural units in the droplets tobe in a substantially more non-equilibrium state than the structuralunits contained within the spray head.

The substrate can be edible to humans, and the coating can be an ediblecoating. The liquid solution within the reservoir can be configured suchthat the structural units in the solute within the reservoir are in asubstantially equilibrium state, such that they do not substantiallybond with one another. The forming of the droplets and the causing ofthe droplets to have a net electrical charge can result in the solutionin the droplets being in a state which is further from equilibrium thana state of the solution contained within the reservoir, allowing thestructural units in the droplets to form covalent bonds to one anotheror to the substrate. The dispensing device can comprise a capillary.

The ion concentration of the liquid solution after the at least partialevaporation of the solvent in the droplets can be within a range that issufficient to catalyze the covalent bonding of the structural unitsadjacent to the droplets on the surface of the substrate. The catalyzingof the covalent bonding can comprise catalyzing polymerization of thestructural units to form a polymeric material structure on the surfaceof the substrate. The catalyzing of the covalent bonding can furthercomprise catalyzing the formation of covalent bonds between at leastsome of the structural units of the polymeric material structure and thesurface of the substrate. The polymeric material structure can be aprotective coating that surrounds the substrate. The method can furthercomprise spraying the droplets on a substrate, wherein the dropletsserve to sanitize the substrate. The substrate can comprise agriculturalequipment, produce, or medical equipment.

The material structure can comprise a polymer formed of the structuralunits. The assembly can further comprise a substrate configured toreceive the droplets, wherein the substrate is edible to humans, and thematerial structure comprises an edible coating. At least some of thestructural units of the edible coating can be covalently bonded to asurface of the substrate.

The details of one or more implementations of the invention are setforth in the accompanying drawings and description below. Other featuresand advantages of the invention will be apparent from the descriptionand drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 are each side views of a deposition system for a coating.

FIG. 3 is a schematic representation of a chemical reaction that leadsto the formation of a coating.

FIG. 4 is a side view of another deposition system for a coating.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Described herein are methods and apparatus for depositing material on orapplying a coating to a substrate. The deposited material or coating isformed from a solution in a reservoir, the solution including a solventwhich contains a solute of monomers and/or oligomers of the material tobe coated over or deposited on the substrate. While in the solvent, themonomers and/or oligomers of the solute are in a non-reactive state, orin a state of low reactivity, such that they do not substantially bondwith one another and/or with other constituents of the solution. Smalldroplets of the solution are then formed and directed towards thesurface that is to be coated. As described in further detail below, themethods by which the droplets are formed cause the monomers/oligomers inthe droplets to become reactive, or to become substantially morereactive than they were in the solution prior to droplet formation. Thuswhen the droplets reach the surface of the substrate, as the solventevaporates (increasing the monomer/oligomer concentration within thedroplets), the activated monomers/oligomers covalently bond directly tothe surface of the substrate or to previously deposited portions (i.e.,previously deposited monomer/oligomer units) of the coating. In thismanner, the coating is “grown” over the surface of the substrate.

As used herein, the term “substrate” refers to any object or material towhich the reactive droplets described herein are directed and over whicha resultant coating is formed or material is deposited. Although in manyapplications the coatings are formed over the entire outer surface ofthe substrate, in some applications the coatings may not cover theentire outer surface or may include apertures or porous regions whichexpose a portion of the outer surface of the substrate.

In some implementations, the substrate is edible to humans, and thecoating is an edible coating. The coating prevents or suppresses waterloss from the substrate via evaporation through the substrate surface(or stem area) as well as oxidation via reaction with oxygen gas thathas diffused into the substrate, and, in addition, it delays the releaseof other volatiles, insulates the produce, and can also help preventsurface abrasion. Examples of edible substrates include fruits,vegetables, produce, seeds, nuts, beef, poultry, and seafood.

In some implementations, the coating is formed as or includes an organicmaterial, while in other implementations the coating is formed as orincludes an inorganic material. In still other implementations, thecoating is formed of or includes both organic and inorganic material.The substrate may be a plant, such as a picked flower, and the coatingmay be a non-toxic coating which serves to increase the lifespan of theplant and prevent spoilage.

Referring now to FIG. 1, a spray head 100 is used to form reactivedroplets 110 of solution and direct them towards the substrate 112. Asused herein, the term “spray head” refers generally to any vessel orcontainer which is capable of holding a solution, the solution being aliquid or fluid, and which causes droplets of the solution to be formedand directed (e.g., “sprayed”) in one or more desired directions. Thespray head 100 includes a reservoir 102 having a dispensing device suchas a capillary 104 on one end. The cross-section of the capillary 104can be circular, square, rectangular, oval, or any arbitrary shape. Thewalls 105 of the capillary 104 are metallic or otherwise formed of anelectrically conductive material, while the walls 103 of the remainderof the reservoir 102 are formed of an electrically insulating material,such as plastic or any other sufficiently rigid, insulating material.

The materials that the walls of the reservoir. 102 and capillary 104 areformed of are chemically inert and should not react with the solution118 contained within the reservoir. The spray head 100 optionallyincludes an annular electrode 108, which is also metallic orelectrically conductive. The capillary 104 and annular electrode 108 areboth configured such that their respective average inner diameters areadjustable. The average inner diameter of the capillary 104 is typicallybetween about 10 microns and 1 meter, and the average inner diameter ofthe annular electrode 108 is typically between about 10 microns and 1meter. The annular electrode 108 is typically substantially centeredaround the tip 106 of the capillary 104, such that the capillary 104 andthe annular electrode 108 may be substantially coaxially aligned. Thespray head 100 can be configured such that the separation between theannular electrode 108 and the tip 106 of the capillary 104 isadjustable. Typically the separation between the annular electrode 108and the tip 106 of the capillary 104 is between about 10 microns and 10centimeters.

Still referring to FIG. 1, the walls of the capillary 104 and theannular electrode 108 are maintained at a different electric potentialby a voltage supply 110 whose output voltage can be varied. As usedherein, a “voltage supply” refers to a device or system that includestwo outputs and generates an electric potential difference between thetwo outputs. A few examples of voltage supplies include batteries,voltage-variable power supplies, photovoltaic or thermoelectricgenerators, gas or diesel generators, and Kelvin generators. Oneterminal of the voltage supply 110 is electrically connected to thewalls of the capillary 104, for example with a conductive wire 114, andthe opposite terminal of the voltage supply 110 is electricallyconnected to the annular electrode 108, for example with anotherconductive wire 116.

As also shown in FIG. 1, the reservoir 102 is filled, or at leastpartially filled, with a solution including a solvent 118 which containsa solute 120 comprised of monomers and/or oligomers of the material tobe deposited over the surface of the substrate 112. The pressure withinthe reservoir 102 is optionally maintained by an external pump 124.

As described above, the solute 120 includes monomers and/or oligomers ofthe material to be deposited over the surface of the substrate 112. Themonomers or oligomers can, for example, be short chain fatty acidspossessing both a carboxyl functionality and a hydroxyl functionality ator near opposite ends of a saturated or unsaturated hydrocarbon chain,which may optionally contain additional chemical constituents (e.g.epoxides, hydroxyls, glycerols, etc.). The monomers may also includeleaving groups bound to one or more of the monomer functionalities,which can be included to influence reaction kinetics and/or altermonomer solubility. The solvent 118 can be any organic or inorganicsolvent which solvates or at least partially solvates themonomers/oligomers that are pumped into the reservoir 102. The solvent118 can be any polar, non-polar, protic, or aprotic solvent, includingany combination thereof. Examples of solvents that can be used includewater, methanol, ethanol, acetone, isopropanol (i.e., isopropylalcohol), or combinations thereof.

During the time that the solution is contained within the reservoir, thesolution is configured such that the monomers/oligomers in the solute120 are in a non-reactive state, or in a state of low reactivity, suchthat they do not substantially bond with one another and/or with otherconstituents of the solution. For example, for some monomer/oligomersolute compositions, the pH of the solution is sufficiently low (i.e.,the solution is sufficiently acidic) to catalyze reactions that resultin covalent bonding of the monomers/oligomers to each other or to thesubstrate. When these monomer/oligomer solute compositions are used, thesolution is configured such that while it is contained within thereservoir, its pH is sufficiently high (i.e., the solution issufficiently neutral/basic) such that the monomers/oligomers in thesolute do not substantially bond with one another and/or with otherconstituents of the solution. For other monomer/oligomer solutecompositions, the pH of the solution is sufficiently high (i.e., thesolution is sufficiently neutral/basic) in order to catalyze thereactions. When these other monomer/oligomer solute compositions areused, the solution is configured such that while it is contained withinthe reservoir, its pH is sufficiently low (i.e., the solution issufficiently neutral/acidic) such that the monomers/oligomers in thesolute do not substantially bond with one another and/or with otherconstituents of the solution.

In view of the configuration of the solution described above, withoutactivating the monomers/oligomers such that they are in a more reactivestate, the monomers/oligomers would be incapable of forming or unlikelyto form covalent bonds to the substrate 112 and/or with each other whenapplied to the substrate, and therefore may not form a coating 113 thatis sufficiently protective. In the case where the pH of the solution isabove the minimum value required to catalyze a reaction, adding an acidto the solution can activate the monomers/oligomers and place them in amore reactive state, such that they are subsequently capable of formingcovalent bonds to the substrate 112 and/or to each other. However, sucha process is undesirable in many applications. For example, inpost-harvest preservation of produce, use of certain strong acids isprohibited in food additives, since such a process can result in theproduce being classified as unsafe for human consumption. Furthermore,once the solute is in a more reactive state, the monomers/oligomersbegin to bond with one another indiscriminately and polymerize prior tobeing placed on the surface of the substrate 112, which can degrade theproperties of the resultant coating 113, or result in the properties ofthe coating 113 being less controllable, as described in more detailbelow. Similarly, in the case where the pH of the solution is below theminimum value required to catalyze reactions, adding a base to thesolution can activate the monomers/oligomers and place them in a morereactive state, such that they are subsequently capable of formingcovalent bonds to the substrate 112. However, similar concerns andcomplications to the case described above where an acid is added to thesolution also persist.

Referring again to FIG. 1, in order to form a coating 113 over thesubstrate 112, the portion of the substrate 112 being coated may besubstantially centered about the tip 106 of the capillary 104 and heldat a controllable distance from the annular electrode 108, and a voltagebetween the annular electrode 108 and the capillary 104 is applied bythe voltage supply 110. The distance between the surface of substrate112 being coated and the annular electrode 108 can be fixed or variedduring the deposition process. Droplets 122 of the solvent 118 andsolute 120 are formed and are directed towards the substrate 112, asshown.

The droplets 122 are formed by applying a voltage applied between theannular electrode 108 and the capillary 104, resulting in a netaccumulation of electric charge along the surface of the solutionadjacent to the tip 106 of the capillary 104. The polarity of the charge(i.e., whether it is positive or negative) is determined by the polarityof the applied voltage. Surface tension effects, in combination with theinduced charge on the surface of the solution at the tip of thecapillary, result in formation of droplets 122. The droplet size andrate of formation can be controlled by adjusting one or more of thefollowing: (1) the voltage and/or current output by the voltage supply110 (thereby adjusting the electric field strength and charge density atthe surface of the solution); (2) the rate of rotation or power of thepump 124 (thereby adjusting the pressure within the reservoir 102); (3)the average radius of the capillary 104; (4) the average radius of theannular electrode 108; (5) the surface tension of the solvent 118;and/or (6) the temperature or conductivity of the solution.

As a result of the applied electric field and resulting chargeaccumulation at the surface of the solution, which causes the droplets122 to form, the droplets possess excess positive or negative charge. Asthe solvent evaporates from the charged droplet 122, the charge in thedroplet leads to further increased or decreased ion concentrationswithin the droplet. For example, in the case where the droplet ispositively charged and the ion is a hydrogen cation, the hydrogen cationconcentration in the droplet increases, resulting in increased dropletacidity. Alternatively, in the case where the droplet is negativelycharged and the ion is hydroxide, the hydroxide ion concentration in thedroplet increases, resulting in increased droplet basicity. This changein ion concentration leads to the formation of reaction intermediatesand causes the monomer/oligomer solute 120 within the droplet to be in anon-equilibrium, and therefore more highly reactive, state.Consequently, when the droplet 122 strikes the surface of the substrate112 and the solvent 118 evaporates (or while it is evaporating), themonomers/oligomers are in an activated, non-equilibrium state, and formcovalent bonds directly to the surface of the substrate 112 or topreviously deposited portions (i.e., previously depositedmonomer/oligomer units) of the coating 113, in an attempt to drive thesystem into a state closer to equilibrium. The process by which themonomer/oligomer units form covalent bonds to the surface of thesubstrate is commonly referred to as surface grafting.

In some implementations, additives such as surfactants, acids, or saltsare added to the solvent 118 in order to adjust the surface tension orconductivity of the solvent and thereby adjust the size and the rate offormation of the droplets 122. In other implementations, weak acids,ions, or non-reactive molecules are added to the solution to control oradjust the properties of the resulting film or coating 113. In stillother implementations, the solution contains pH stabilizers ormodifiers. In yet other implementations, the solution includesadditional materials which are also transported to the surface of thesubstrate 112 in droplets (or are deposited separately) 122 and aresubsequently encapsulated by the coating 113, i.e., the coating 113 isformed at least partially around the additional material. Examples ofsuch additional materials include cells, biological signals, vitamins,minerals, pigments, aromas, enzymes, catalysts, and time-release drugs.The additional materials can be non-reactive materials, can be reactivewith both the coating 113 and the surface of the substrate 112, can bereactive with the surface of the substrate 112 but not with the coating113, or can be reactive with the coating 113 but not with the surface ofthe substrate 112.

In some implementations, oxidation or reduction reactions take place inthe droplets in the solution while the droplets are in or near the sprayhead, forming reactive intermediates or leading to the formation of newcompounds which subsequently react with the substrate. For example, asthe solvent evaporates from the charged droplet 122 prior to the dropletreaching the surface of the substrate, ion concentrations in the dropletincrease or decrease, and the state of the solution in the dropletshifts further towards non-equilibrium relative to solution in thereservoir. This drives the formation of reactive intermediates or newcompounds, since forming these reactive intermediates or new compoundsrepresents a new equilibrium state within the droplets. An example ofthis could include the formation of reactive derivatives of carboxylicacid functionalities such as acyl halides (e.g. acyl chloride) thatreact with alcohols or amines to form esters of amines. Other examplesof reactive intermediates include carbocation formation under highhydrogen cation concentrations within the droplets. In still otherimplementations, the reactive intermediate is a salt.

For many applications, it is desirable that the pH of the solution bevery close to the threshold required to activate the monomers/oligomersin the solute 120 while the solution is contained in the reservoir 102,prior to droplet formation. This is because the change in pH followingdroplet formation may be small, so if the pH of the solution prior todroplet formation is not sufficiently close to the threshold value, thechange in pH following droplet formation may not be adequate to catalyzecertain types of polymerization reactions. For example, if a minimum pHis required to activate the monomers/oligomers and catalyze reactions,the pH of the solution can be slightly below this minimum threshold pHvalue (i.e., between 0.5 and 0.999 times the threshold pH value). If apH below some maximum value is required to activate themonomers/oligomers and catalyze reactions, the pH of the solution can beslightly greater than this maximum threshold pH value (i.e., between1.001 and 2 times the threshold pH value). For example, if a pH below 5is optimal for catalyzing reactions, the pH of the solution prior todroplet formation can be between about 5.1 and 8. In some applicationswhere a minimum pH is required to activate the monomers/oligomers andcatalyze a reaction, the pH of the solution prior to droplet formationcan be less than 0.9 times or less than 0.8 times this minimum thresholdpH value. In some applications where the pH required to activate themonomers/oligomers and catalyze the reaction must be below some maximumthreshold pH value, the pH of the solution prior to droplet formationcan be greater than 1.1 times or greater than 1.3 times this maximumthreshold pH value.

The pH of the solution while it is contained in the reservoir 102 can beadjusted by the addition of acids or bases to the solution, eitherbefore or after it is placed in the reservoir. Hence, although thesolution may be free of strong acids and bases, in some cases reasonablysmall quantities of acids and bases may be added to or included in thesolution in order to better tune the pH of the solution. The ability toexternally modify solution pH with the application of an external fieldenables molecules with very high acid dissociation constant (pKa) values(for example, above 15.5) to be added to the solution in the reservoirwithout substantially catalyzing reactions within the reservoir prior todroplet formation, while still allowing for reactions to occur at thesurface of the substrate after the droplets are incident on thesubstrate.

Referring now to FIG. 2, an alternative configuration for a spray head200 is shown. Spray head 200 of FIG. 2 is similar to spray head 100 ofFIG. 1, except that annular electrode 108 shown in FIG. 1 does not needto be included (although it may optionally still be included). Instead,in spray head 200, one terminal of the voltage supply 110 iselectrically connected to the walls 105 of the capillary 104, forexample with a conductive wire 114, and the opposite terminal of thevoltage supply 110 is electrically connected to a charge source 208. Thecharge source 208 can for example be an electrical ground or a DCvoltage plane. The charge source 208 supplies/sinks electric chargeto/from the capillary 104 when a voltage is applied by the voltagesupply 110. Similar to the case of spray head 100, the applied voltagebetween the capillary 104 and the charge source 208 results in a netaccumulation of electric charge along the surface of the solutionadjacent to the tip 106 of the capillary 104. The polarity of the charge(i.e., whether it is positive or negative) is determined by the polarityof the applied voltage. Capillary surface tension effects, incombination with the charge on the surface of the solution at the tip ofthe capillary 104, result in the droplets 122 being formed. This mode ofoperation for the spray head is known as conduction mode, while the modeof operation in which the voltage is applied between the capillary 104and an annular electrode 108 (as in FIG. 1) is known as induction mode.

In some implementations, the monomers/oligomers of the solute 120 arebifunctional or polyfunctional molecules. A bifunctional orpolyfunctional molecule is one which includes two (in the case ofbifunctional) or multiple (in the case of polyfunctional) bonding sitesfor forming covalent bonds with other atoms or molecules. As such,bifunctional and polyfunctional molecules are each able to form covalentbonds with multiple other atoms or molecules. Whenbifunctional/polyfunctional molecules are implemented as themonomers/oligomers of the solute 120, the coating 113 forms as follows.Some or all of the first reactive monomers/oligomers to reach thesurface of the substrate 112 form covalent bonds to the substrate at abonding site at the surface of the substrate (assuming the substrate'ssurface is composed of a material capable of reacting with the incidentreactive intermediate). Because each of the monomers/oligomers includemultiple bonding sites, subsequent monomers/oligomers form a covalentbond either to the substrate 112 at a non-occupied bonding site at thesurface of the substrate, or to a non-occupied bonding site of one ofthe already bound monomers/oligomers. In this way, a polymer filmcoating is “grown” over the surface of the substrate, where asubstantial number of the monomer/oligomer units of the coating 113 arecovalently bonded to one or more neighboring molecules, and the entirecoating 113 is tightly bound to the surface of the substrate 112.

Coatings formed by the polymerization reaction described above can havehigh structural integrity and very low permeability, making them idealfor reducing water loss and oxidation in produce and other edibleproducts, while also helping prevent surface abrasion. Because strongacids are not used to catalyze elimination reactions during the coatinggrowth process, the coatings can be edible and safe to consume. Thus, insome applications where the coatings are applied to edible products, thecoatings can allow for extended transport of the edible products withoutor with reduced need for refrigeration, and the coatings can be safelyconsumed along with the edible products.

Various properties of coatings formed by the polymerization reactiondescribed above can be tuned by adjusting various parameters of thedeposition precursor, as well as parameters of the components of thespray head 100. For example, the rate of ripening of post-harvest fruitand produce can be adjusted by tuning the cross-link density of thepolymer coating, its thickness, and/or its composition. Polymer coatingsgrown from bifunctional or polyfunctional monomer units tend to havehigher cross-link densities than those grown from bifunctional orpolyfunctional oligomer units (e.g., dimers, trimers, tetramers,polymers, etc.), thereby resulting in slower rates of ripening,transpiration, and/or senescence. Hence, when slow ripening rates aredesired, solutes formed of monomer units can be used, while if fasterripening rates are desired, solutes formed of oligomer units can beused.

When monomer units are used, the cross-link density can be decreasedslightly by allowing some of the monomers to react with one another andform oligomer chains prior to reacting at the surface of the substrate112. For example, in larger droplets 122 (or at higher reservoirconcentrations) that contain larger numbers of monomer units, there is ahigher probability of monomers bonding with one another to form oligomerunits prior to reacting with the surface of the substrate 112. Hence,larger droplets, higher monomer concentrations in the solution withinthe reservoir, as well as a larger separation between the tip 106 of thecapillary 104 and the substrate 112, tend to lead to lower cross-linkdensities.

Furthermore, because the solvent in the droplets 122 evaporates whilethe droplets are traveling between the spray head and the substrate, theconcentration of the solute incident on the surface of the substrate 112can be tuned by adjusting both the size of the droplets and the distancethat the droplets travel through the air. As previously described, thedroplet size and rate of formation can be controlled by adjusting thevoltage that is output by the voltage supply 110 (thereby adjusting theelectric field strength), the rate of rotation of the pump 124 (therebyadjusting the pressure within the reservoir 102), the average radius ofthe capillary 104, the average radius of the annular electrode 108,and/or the surface tension, conductivity, or temperature of the solvent118.

In some implementations, the monomers/oligomers of the solute 120 aremonofunctional molecules. A monofunctional molecule is one which onlyincludes a single bonding site for forming a covalent bond with anotheratom or molecule. As such, once a monofunctional molecule has formed acovalent bond with another atom or molecule, it will not participate infurther reactions. When the monomers/oligomers of the solute 120 aremonofunctional molecules, after being transported to the surface of thesubstrate 112 in a charged droplet 122, the reactive monofunctionalmolecules each form a covalent bond to a bonding site on the surface ofthe substrate 112, after which they are unable to participate in furtherreactions. As more reactive monofunctional monomers/oligomers aretransported to the surface of the substrate 112, they covalently bond tobonding sites of the substrate surface until all of the bonding sitesare occupied. Thus, the resultant coating is comprised of a monolayer ofthe monomer/oligomer constituents, rather than being a polymer film.Such films are typically more permeable than films formed by thepolymerization reactions previously described, making them ideal forapplications where faster ripening is desirable. Additionally, thesemonolayer films typically require smaller quantities of precursorsolution and can be formed in a shorter time than the polymer filmspreviously described, resulting in reduced cost as compared to the costof creating the polymer films. Inclusion of such monofunctional units inthe deposition process may also be used to control crosslinking densityof deposited bifunctional/polyfunctional monomers/oligomers by reducingthe effective number of functional units capable of covalent bonding, asthe reaction with monofunctional units reduces the number of availablegrafting sites for newly deposited monomers/oligomers.

FIG. 3 is a schematic representation of an example chemical reactionthat leads to the formation of the organic coatings described above. Inthis example, the substrate or previously deposited material is a linearpolyester chain represented by the formula 310 in FIG. 3. This polymerchain is covalently bound by ester linkages and is polyfunctional,containing one carboxylic acid functionality and n+1 hydroxylfunctionalities. The addition of a polyfunctional hydroxy fatty acidchain (shown in formula 320) in the presence of an acid catalyst resultsin the formation of an ester bond between the hydroxyl group of thelinear polyester chain and the carboxylic acid group of thepolyfunctional hydroxy fatty acid via an elimination reaction in whichwater is the byproduct (shown in formula 330).

Although FIGS. 1 and 2 each illustrate a single spray head 100 or 200being used to apply a coating 113 to a substrate 112, a plurality ofspray heads (i.e., at least 2, as shown in FIG. 4), at least 3, or atleast 4 spray heads configured in an array (not shown) can be used toapply the coating 113. In this case, the monomers/oligomers in theactivated droplets from each spray head 400 are able to react with andform covalent bonds to the substrate 112 and to other activatedmonomer/oligomer units from any of the spray heads 400 in the array.While in some cases the solution composition in each of the spray headsis similar or substantially the same, in other cases the solutioncompositions in different spray heads of the array may be different. Incomparison to using a single spray head, such an array can reduce thetime required to coat a substrate and can allow for large scaleproduction of the coatings 113, as described herein.

In addition to forming coatings that surround or at least partiallysurround a substrate, the spray heads of FIGS. 1, 2, and 4, along withthe associated methods described herein, can also be used to form othertypes of material structures. For example, the monomer/oligomer unitscan be used as “ink” in three dimensional printing systems. That is, athree dimensional material structure can be built from monomer/oligomerunits delivered to a substrate using the systems and methods describedabove. Such printed three dimensional polymer structures can be veryinexpensive to fabricate, and can have a degree of structural integritywhich can be modified in-situ due to the ability to form and control thedensity of tight covalent bonds formed between the adjacentmonomer/oligomer units. In this application, the monomer/oligomer unitsmay be reactive with one another, but not with the substrate on whichthey are deposited, such that the resulting printed three dimensionalstructure may easily be separated from the substrate.

Alternatively, the monomer/oligomer units may be reactive with oneanother and also with the substrate on which they are deposited, suchthat the printed three dimensional structure is formed as an extensionof the substrate. An example of such an application includes printingtread directly onto a tire (such as a bicycle or automobile tire made ofsuitable material), where the tire is smooth prior to the printing ofthe tread but includes a tread pattern after printing. As anotherexample, the printed structures can be used as scaffolds for biologicalmaterials such as cells or proteins. In this example, biocompatiblemonomer units may be deposited layer by layer or “printed” into adesired shape which may then serve as a scaffold for cell growth. Thistechnique has the added advantage that during deposition, peptidesequences, proteins, or other biological signaling molecules may becovalently bound to specific positions of the scaffold, thus allowingdirected cell growth on the scaffold.

In still another variation of the systems and methods described herein,the solution in the spray head reservoir does not contain solutemolecules which are capable of undergoing polymerization reactions.Instead, the solution may contain only solvent molecules, ions, acids,and/or other highly non-polymerizable molecules. In this variation, thedroplets which are formed contain no monomer/oligomer units. The fieldgenerated by the voltage supply and the associated charging of thedroplets can change the ion concentration or the acidity or basicity ofthe solution in the droplets, as compared to the solution in thereservoir prior to droplet formation. When the droplets are directedtowards a substrate, they can catalyze reactions (e.g. polymerization ordepolymerization) between molecules already on the substrate. Thisalternative variation may also be used to modify the substrate surfaceprior to coating deposition. That is, acidified or basified solventdroplets without any monomer/oligomer solute can first be applied to thesubstrate in order to modify the surface of the substrate, followed byapplication of droplets containing both solvent and monomer/oligomersolute in order to form a coating or other three-dimensional structure.Alternatively, such a process could be used to sanitize agriculturalequipment, produce surfaces, or medical equipment, for example, withoutthe use of more toxic chemicals.

The methods described herein allow for the formation of charged dropletsof controllable solvent/solute compositions and concentrations with acontrollable level of acidity or pH or the formation of alternativereactive intermediates (e.g. acyl halides). In some implementations,these methods provide a physicochemical technique by which to catalyzeelimination reactions without the addition of strong acids. Furthermore,the methods described herein can allow for the controllable delivery ofdroplets containing activated monomers/oligomers that participate inreactions (e.g. elimination reactions) once they are deposited on asubstrate.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the techniques and devices describedherein. For example, although in FIGS. 1, 2, and 4 the solution ismaintained in the reservoir of one or more spray heads prior to dropletformation, the solution may be maintained in an external reservoir andprovided to the spray head, for example through a tube, or by drippingout of the reservoir, as additional solution is needed. Additionally,while in FIGS. 1, 2, and 4 the droplets are formed and made to bereactive by a voltage applied between the capillary and either theannular electrode or an electrical ground, other methods of formingactivated or highly reactive droplets can be implemented. For example,rather than applying a voltage directly to or between portions of thespray head, an external electric field can be created, and the surfaceof the solution at which the droplets are formed is placed within theelectric field. Or, rather than using an electric field to induceformation of reactive droplets, heat or photoexcitation could be used.That is, the surface of the solution at which the droplets are formedcan be heated, or photons can be directed onto the surface, in order toinduce formation of activated droplets. Other processes may also beemployed to help steer the droplets onto the surface, for example aircurrents or additional electric fields. Features shown in each of theimplementations may be used independently or in combination with oneanother. Accordingly, the invention is only limited by the scope of thefollowing claims.

What is claimed is:
 1. A method of forming a coating over a surface of asubstrate from structural units contained within a liquid solution, theliquid solution comprising a solvent, the method comprising: formingpositively charged droplets of the liquid solution, wherein the dropletsinclude a plurality of the structural units; directing the droplets tothe surface of the substrate while allowing at least a portion of thesolvent within the droplets to evaporate, thereby increasing a positivecharge concentration within the droplets; and forming the coating overthe surface of the substrate from the plurality of the structural unitsin the droplets; wherein the increase in the positive chargeconcentration within the droplets as the droplets evaporate causes thestructural units to be in a more highly reactive state, thereby allowingthe structural units in the droplets to form covalent bonds to oneanother or to the surface of the substrate.
 2. The method of claim 1,wherein the liquid solution further comprises an additive selected fromthe group consisting of a surfactant, an acid, and a salt.
 3. The methodof claim 1, wherein the coating is formulated to prevent or suppresswater loss or uptake by the substrate, volatile loss or uptake by thesubstrate, oxidation via reaction with oxygen gas that can diffuse intothe substrate, or surface abrasion.
 4. The method of claim 1, whereinthe positively charged droplets are formed by applying a voltage betweenan electrode of a spray head containing the liquid solution and acapillary of the spray head.
 5. The method of claim 1, wherein thesubstrate is edible to humans, and the coating is an edible coating. 6.The method of claim 1, wherein the structural units comprise monomerunits, oligomer units, or combinations thereof.
 7. A method of forming acoating over a surface of a substrate from structural units containedwithin a liquid solution, the liquid solution comprising a solvent, themethod comprising: forming negatively charged droplets of the liquidsolution, wherein the droplets include a plurality of the structuralunits; directing the droplets to the surface of the substrate whileallowing at least a portion of the solvent within the droplets toevaporate, thereby increasing a negative charge concentration within thedroplets; and forming the coating over the surface of the substrate fromthe plurality of the structural units in the droplets; wherein theincrease in the negative charge concentration within the droplets as thedroplets evaporate causes the structural units to be in a more highlyreactive state, thereby allowing the structural units in the droplets toform covalent bonds to one another or to the surface of the substrate.8. The method of claim 7, wherein the liquid solution further comprisesan additive selected from the group consisting of a surfactant, an acid,and a salt.
 9. The method of claim 7, wherein the droplets serve tosanitize the substrate.
 10. The method of claim 9, wherein the substratecomprises agricultural equipment, produce, or medical equipment.
 11. Themethod of claim 7, wherein the structural units comprise monomer units,oligomer units, or combinations thereof.
 12. A method of forming acoating over a surface of a substrate from structural units containedwithin a solvent of a liquid solution, the liquid solution furthercomprising an enzyme, the method comprising: forming droplets of theliquid solution, wherein the droplets include a plurality of thestructural units and the enzyme; directing the droplets to the surfaceof the substrate; and forming the coating over the surface of thesubstrate from the plurality of the structural units in the droplets;wherein the forming of the coating comprises allowing the structuralunits in the droplets to form covalent bonds to one another or to thesurface of the substrate; the enzyme is reactive with the coating orwith the surface of the substrate; and the substrate is edible and thecoating is an edible coating.
 13. The method of claim 12, wherein thestructural units comprise monomer units, oligomer units, or combinationsthereof.
 14. The method of claim 13, wherein the coating comprises apolymer formed from the structural units.
 15. The method of claim 13,wherein the monomer units, oligomer units, or combinations thereofcomprise fatty acids.
 16. The method of claim 12, wherein the enzyme isreactive with the coating but not with the surface of the substrate. 17.The method of claim 12, wherein the coating prevents or suppresses waterloss from the substrate.
 18. The method of claim 12, wherein the ediblesubstrate is selected from the group consisting of fruits, vegetables,produce, seeds, nuts, beef, poultry, and seafood.
 19. A method offorming a coating over a surface of a substrate from a plurality ofstructural units, the structural units comprising monomer units,oligomer units, or combinations thereof, the method comprising:transporting the structural units onto the surface of the substrate;transporting an enzyme onto the surface of the substrate; and formingthe coating over the surface of the substrate from the plurality of thestructural units; wherein the forming of the coating comprises allowingthe structural units to form covalent bonds to one another or to thesurface of the substrate; and the substrate is edible and the coating isan edible coating.
 20. The method of claim 19, wherein the enzyme isreactive with the coating or with the surface of the substrate.
 21. Themethod of claim 19, wherein the enzyme is reactive with the coating butnot with the surface of the substrate.
 22. The method of claim 19,wherein the enzyme is reactive with the surface of the substrate but notwith the coating.
 23. The method of claim 19, wherein the forming of thecoating comprises polymerization of the structural units, wherein thepolymerization occurs directly on or adjacent to the surface of thesubstrate.
 24. The method of claim 19, wherein the monomer units,oligomer units, or combinations thereof comprise fatty acids possessinga carboxyl functionality and a hydroxyl functionality.
 25. The method ofclaim 19, wherein the monomer units, oligomer units, or combinationsthereof comprise a saturated or unsaturated carbon chain.
 26. The methodof claim 25, wherein the monomer units, oligomer units, or combinationsthereof further comprise a glycerol constituent.
 27. The method of claim19, wherein the coating prevents or suppresses water loss from thesubstrate.
 28. The method of claim 19, wherein the edible substrate isselected from the group consisting of fruits, vegetables, produce,seeds, nuts, beef, poultry, and seafood.